Vortex generator devices are used to delay or eliminate flow separation along a flow control surface. Conventional vane type vortex generators are used widely on aircraft for lift enhancement and drag reduction. Vane type vortex generators are sharp blades extending normal to the surface and with an angle of attack to the flow at least during the time vortices are to be generated. Flow migrating over the outer tip of each vane can create a useful streamwise vortex that mixes high speed fluid from the free stream into the sluggish energy-deficient boundary layer. This mixing increases the energy in the boundary layer, making it resistant to flow separation. The process is called passive boundary layer control. Vane vortex generators are disadvantageous when they are needed for only a small part of the operating time because they are high drag devices. Therefore, in some applications, the complexities of making them retractable has been considered, such as is shown in Bauer, U.S. Pat. No. 4,039,161.
Low-drag vortex generators such as the ramp vortex generators shown in STEPHENS, U.S. Pat. No. 2,800,291, the crest vortex generators shown in KUETHE, U.S. Pat. Nos. 3,578,264 and 3,741,285, and the cascade vortex generators shown in WHEELER, U.S. Pat. No. 4,455,045 employ rather passive geometries that do not themselves operate at an appreciable lift coefficient, so they minimize induced drag.
To minimize parasitic drag requires that any vortex generator be sized so it is submerged entirely within the boundary layer. This results in weak vortex formation because the vortices are formed from sluggish boundary layer flow. Consequently, when sized to fit within the boundary layer, neither vane vortex generators, nor STEPHENS' ramps have proved to be powerful enough to be useful in practical applications.
KUETHE's crest vortex generators and WHEELER's cascade vortex generators address this problem of vortex formation from sluggish boundary layer flow by employing fore and aft arrays of vortex generators so that upstream vortices are reinforced and augmented by uniting them with downstream formed vortices of the same sign of rotation. Unfortunately, this approach is fraught with difficulty. For example, with KUETHE's crests, any off-axis flow from yaw or side currents carry the upstream vortices away at an angle so they are likely to arrive at the wrong downstream stations and create damaging vortex interference. WHEELER's cascades avoid this problem because the devices physically overlap themselves, but their complex geometry is laborious to fabricate and costly to tool or machine. Moreover, there are many applications with insufficient room to accommodate the chordwise length of the cascades.
Therefore there is need for an easily fabricated, passive device which can be sized to nestle within the boundary layer and which can pump large amounts of energy into the boundary layer of a flowing medium to prevent flow separation with its inevitable drag increase and lift reduction. There also is need for an easily installed device which can be sized to restrict spanwise flow even at a flow surface tip where undesirable tip vortices normally form when the flow surface is generating lift.